Uremic toxins are all substances which have to be excreted renally. These uremic toxins accumulate in the body of a human being/patient having lacking/reduced renal function. The kidney substitution therapy, for instance, in the form of a hemodialysis (HD) therapy, offers the possibility of treating the patient in that uremic toxins from the intracorporeal circulation of the patient are transferred to a circulation with dialysis fluid with a dialyzer. In this process, the patient's blood is guided via an extracorporeal blood circulation into the dialyzer at the blood side thereof and is subsequently resupplied into the patient's intracorporeal blood circulation. At the other dialysate side the dialyzer is supplied through an inlet for dialysate with fresh dialysate which absorbs the uremic toxins. The spent dialysate finally leaves the dialyzer through an outlet for dialysate.
The dialysate is in general an aqueous bicarbonate buffer solution having properties similar to those of blood with respect to the pH value and the electrolyte composition. In the course of a hemodialysis therapy the dialysate gets into contact with a patient's blood via a membrane within the dialyzer. Uremic toxins in the range of 18-65k daltons may thus diffuse in the dialyzer from the patient's blood into the dialysate, so that fresh dialysate at the inlet becomes the spent dialysate at the outlet.
The quality of a blood purification therapy such as a dialysis therapy may be assessed in that the quantity of uremic toxins in the dialysate is detected before and after the treatment. It is, however, not only important to obtain knowledge about the loading of the spent dialysate with uremic toxins as such, but the knowledge about the composition of the uremic toxins may also be of interest. With some uremic toxins such as creatinine it is namely possible to make statements, for instance, with respect to a patient's nutritional status. Therefore, both the absolute quantity of extracted uremic toxins and their composition are of significance.
Some uremic toxins absorb light in the range between 190 nm and 350 nm. The absorption may be utilized to determine the concentration of uremic toxins in the dialysate at the outlet side. The absorption of light may be measured with optical sensors, e.g. with optical absorption sensors such as they are sufficiently known from the state of the art. Optical absorption sensors are devices determining the absorption of a substance or of a substance mixture on the basis of a transmission measurement. With the aid of the Bouguer-Lambert-Beer law at one or a plurality of optical wavelengths it is also possible to determine the concentration of absorbing compounds (e.g. creatinine, uric acid, etc.), for instance, in optically clear fluids. Although the absorption sensor physically measures a transmission, the signal is often output as a decadic absorption measure. The decadic absorption measure is proportional to the molar extinction coefficient, the concentration and the optical path length. In general, the path length is predetermined by the sensor geometry and is thus a constant. The same applies to the molar extinction coefficient which is a specific substance property. For a given wavelength it is constant. In the end, the decadic absorption measure in a sensor is directly proportional to the only remaining variable factor, namely the concentration of a marker. This, however, applies only to solutions with one single dissolved substance.
The difficulty arising during the examination of solutions with a plurality of components is, however, the distinct allocation of an absorption signal to the concentration of a component. This is particularly the case if the absorption bands of the interesting substances overlap each other (such as, for instance, creatinine and uric acid) or with uninteresting substances.
When it comes to the therapy-accompanying determination of a measure for dialysis quality, the qualitative measure Kt/V is presently used, with                K clearance (is determined e.g. by the urea content of the blood before and after the dialysis—the clearance results in particular from a comparison of the concentration of urea at the blood inlet with the concentration of urea at the blood outlet of the dialyzer)        t effective dialysis time in minutes, and        V the part of the body volume in which urea is distributed        
Since this measure is qualitative, it does not allow any statements about the actually removed quantities of a particular substance such as e.g. creatinine. The market, however, demands also a quantitative measure of a selected substance which renders it possible to make further statements about the patient. Thus, the duration of the dialysis may still be controlled without problems via the removed total quantity of a marker, but additionally a trend about several therapies may be made which allows assessing the nutritive state of the respective patient, for instance, in the case of creatinine.
Basically there exist methods under laboratory conditions, in particular enzymatic tests, which are capable of determining creatinine concentrations in serum. However, e.g. one or two measurements of the creatinine concentration in the blood are not sufficient for determining the removed quantity of creatinine in quantitative respect. This is substantially due to the fact that the removal function, i.e. the function indicating the concentration in the blood or in the dialysate during a dialysis, is unknown.
Therefore, a higher-frequent measurement of the creatinine concentration in the blood or in the spent dialysate is required. The measurement in the spent dialysate has turned out to be advantageous as compared to the measurement in the blood. The spent dialysate always contains the actually removed quantity of creatinine as compared to blood in which various processes cause the rediffusing of creatinine and thus distort the determination of the actually removed quantity.